Gas-purifying composition and method of producing it

ABSTRACT

In removing nitrogen oxide from a gas containing it, the gas is passed through a region where at least one of calcium sulfate and calcium hydroxide coexists with powerful oxidation agents such as sodium chlorite, potassium permanganate, etc. thereby removing the nitrogen oxide, mainly nitrogen monoxide, from the gas. This method is very effective in purifying exhaust gases resulting from burning operations, since 80 to 90% or more of the nitrogen oxide contained in the exhaust gases is nitrogen monoxide. In addition, this method is also effective in purifying indoor air.

This application is a division of Ser. No. 516,869, filed Oct. 21, 1974. Now U.S. Pat. No. 4,025,603.

The present invention relates to methods for purifying a gas by removingnitrogen oxide, mainly nitrogen monoxide, from the gas containing thenitrogen oxide harmful to human bodies; and to compositions usedtherefor. Especially, the present invention is intended to purifyexhaust gas from burning operations and to purify indoor air. In recentyears, atmosphere pollution has been caused by gases emitted fromvarious types of combustors. Especially, photochemical smogs have becomea big problem. The nitrogen oxide component of these gases produces aserious problem. 80 to 90% or more of the nitrogen oxide contained inthe gases emitted from the combustor is nitrogen monoxide. Removal ofthe nitrogen monoxide is a big problem yet to be solved.

Conventionally, there have been known a contact reduction method, anadsorption method, an absorption method, etc. as methods for removingthe nitrogen oxide from the gases. However, there have been manyproblems in practical uses thereof. Namely, according to the contactreduction method, reducing agents such as CO, NH₃, hydrocarbons, H₂ S,etc. are used. The reducing agents are reacted upon NO in the gas, and2N is taken out of 2NO to produce N₂ gas. In this contact reductionmethod, reaction at high temperatures is necessary. In a case where muchoxygen remains in the exhaust gases, the reducing agents are consumed.There are also problems in that reducing NO into N₂ is difficult toeffect, among other problems. According to the adsorption method,adsorbents such as activated charcoal, silica gel, etc. are used. Theexhaust gas is brought into contact with the adsorbents to physicallycause the nitrogen oxide to adhere to the adsorbents. This adsorptionmethod has disadvantages in that the adsorption of NO is difficult,adsorption capacity being low, and adsorption efficiency being lowespecially at higher temperatures, among other disadvantages. Accordingto the absorption method, absorbents such as oxidation agent, chemicalreaction agent, water, alkali solution, etc. are used. The absorbentsare brought into contact with the gas to cause the adsorbents to absorbthe nitrogen oxide in the gas for removal thereof from the gas. Thisadsorption method had disadvantages in that any apparatus used thereformust be large on a plantlike scale, and the absorbents have their ownproblems. For example, in using the oxidation agent, the reactionthereof is reliable. However, as the absorbents are higher in price,they must be reclaimed and recovered. Treatment of by-products is hardto effect. In a case where sulfuric acid is used as the chemicalreaction agent, the process becomes complex, while in a case whereferrous-sulfate is used, oxygen remaining in the gas is apt to reactmore than the nitrogen oxide, whereby reaction of the nitrogen oxide isinterfered with. Also, in a case where water and alkali solution areused, NO is difficultly absorbed. Accordingly, a process of oxidizingthe NO to NO₂ is required as a pretreatment. Furthermore, in case ofwater, high temperature gases of 100° C. or more cannot be introduced.

Also, a majority of the conventional methods has been intended to removenitrogen dioxide. In the exhaust gases from actual combustion,generation of nitrogen monoxide represents an overwhelming majority.Accordingly, removal of the nitrogen monoxide is more important.Therefore, conventionally, in order to remove the nitrogen monoxide, atfirst, the pre-processing for oxidizing NO into NO₂ is required to beeffected. Thereafter, the adsorption and removal operation is effectedby the adsorbents, or by the alkali aqueous solution. However, thesemethods are out of practice in that the removal capacity thereof islimited, and the apparatus therefor is bigger on scale.

Also, according to Japanese Patent Publication No. 10,048/1968, there isa method of removing the nitrogen oxide contained in the gases, which isdry and easy to use. This method comprises the steps of adhering one ormore of FeSO₄, (NH₄)₂ SO₄, PdSO₄, KMnO₄, KClO₃, NaClO, NaClO₂, Na₂ MoO₄,K₂ S₂ O₈, Na₂ S₂ O₂, Na₂ HPO₄, Na₂ O₂, As₂ O₃, CuCl₂, and IrCl₃ ontoactivated aluminum porous carriers such as activated alumina, activatedbauxite, activated silica alumina gel, etc., preferably by adhering themtogether with acid or alkali, and passing the gas containing thenitrogen oxide through the dried mixture as a packed layer thereby toremove the nitrogen oxide from the gas. However, this method is notsufficient in terms of removal rate and removal capacity.

It is a primary object of the present invention to provide compositionswhich have a high rate and capacity of removing nitrogen oxide,especially nitrogen monoxide, from gases, and to provide methods ofremoving the same. Namely, as the nitrogen oxide contained in theexhaust gas resulting from combustion is 80% or more nitrogen monoxide,such a higher removal rate and capacity are very useful in purifying theexhaust gas.

It is another object of the present invention to provide compositionsand removal methods which can be of a dry type easy to use, and whichare higher in removal rate and capacity with respect to nitrogen oxide,as a dry type. In constructing an exhaust gas purifying apparatus, thisfact is helpful in simplifying the structure thereof.

It is a further object of the present invention to provide compositionsand removal methods by which nitrogen oxide of a low concentration inthe atmosphere or environment can be better removed. This is useful inpurifying the air in places where many people gather such as waitingrooms, town halls, theaters, schools, etc., or in houses.

Namely, the present invention has been developed mainly with an objectto purify exhaust gas from combustion, and indoor air. Especially,experiments have been effected for comparatively small combustors suchas household boiler, gas water boiler, etc. as combusting apparatus.Accordingly, as the purifying apparatus is required to be simple inconstruction, easier to use, and lower in cost, the development thereofhas been effected with particular stress on higher capability of the drytype of apparatus. However, the present invention can be applied even toa wet type of apparatus using a packed tower, etc.

The feature of the present invention is that the gas containing thenitrogen oxide is passed through a region where at least one of calciumsulfate and calcium hydroxide co-exist together with a powerfuloxidation agent for oxidizing NO to NO₂ thereby to remove the nitrogenoxide in the gas. At this time, the powerful oxidation agent reacts notonly with the NO in the gas, but also the NO₂ etc. to absorb them, andremoves the nitrogen oxide from the gas. On the other hand, the calciumsulfate and/or the calcium hydroxide serve as a carrier for the powerfuloxidation agent, and function to widen the contact area of the powerfuloxidation agent with the NO, etc. due to their porosity and largerspecific surface area. It seems that their own pH, etc. produceinfluences upon removal of the nitrogen oxide. It is effective to usesodium chlorite or potassium permanganate especially as the powerfuloxidation agent. Concretely, various combinations such as calciumsulfate and sodium chlorite; calcium hydroxide and sodium chlorite;calcium sulfate, calcium hydroxide and sodium chlorite; calcium sulfateand potassium permanganate; calcium hydroxide and potassiumpermanganate; calcium sulfate, calcium hydroxide and potassiumpermanganate; and others are considered. Furthermore, it is alsopossible to add the other addition agent thereto. As embodiments, thereare a dry method of mixing the calcium sulfate and/or the calciumhydroxide with the powerful oxidation agent thereby to constitute a gaspermeable mixture such as a packed layer, porous body, etc. by use of asolid-like composition obtained through a proper process, and passingthe gas through the gas permeable mixture; a method of using afluid-like composition in which the above-mentioned composition has beensuspended in water, etc. thereby to blow the gas into the fluid-likecomposition; a method of spreading a solution of the powerful oxidationagent on the packed layer composed of the calcium sulfate and/or thecalcium hydroxide to pass the gas through the packed layer; and othermethods. However, upon consideration of practicality such as easieroperation, etc., the dry method is best suitable.

FIGS. 1 and 2 show a diagram of an experimental apparatus respectively.

FIGS. 3 and 4 are graphs each showing variations in NO removing rate ofeach kind of absorption liquid as time lapses.

FIG. 5 is a graph showing service life test results of an absorptionliquid which is especially superior in removal efficiency.

FIG. 6 is a graph showing comparisons of NO removing rate in a casewhere an NaClO₂ -containing composition of the present invention and theconventional composition are applied to a combustion exhaust gas.

FIGS. 7, 8 and 9 are graphs concerned with NaClO₂ and showing results asto how NO removal efficiency changes with changes in each kind ofcomposition.

FIG. 10 is a graph showing the NO removing rate in a case where theNaClO₂ -containing composition of the present invention is applied to acombustion exhaust gas.

FIG. 11 is a graph concerned with KMnO₄ and showing results as to howthe NO removal efficiency changes with changes in each kind ofcomposition.

FIG. 12 is a graph showing the NO removal rate in a case where the KMnO₄-containing composition of the present invention is applied to thecombustion exhaust gas.

FIG. 13 is a graph showing variations in NO removal rate with timelapse, centered on KMnO₄.

FIG. 14 is a view showing results as to how the mixing ratio of waterand ethanol produces effects upon the NOx removal in order to estimatethe aptitude as a binder for extrusion granulating operation.

FIG. 15 is a graph showing correspondence of the NO and SO₂ removingefficiency to wind speeds in a case where the composition of the presentinvention is used as an air purifying filter.

FIG. 16 is a graph showing correspondence of NO removal efficiency andpressure loss to wind speeds in a case where the composition of thepresent invention is carried on a gas permeable construction member toserve as a filter.

As described hereinabove, in removing the nitrogen oxide contained inthe gas, it is a principal object to remove the NO. In order to removethe NO, there is generally employed an absorption method of reacting theNO upon the absorbents. Accordingly, application of each kind ofmaterial as an absorbent of NO has been investigated. Namely, in orderto develop a practical removal process concerning the absorption andremoval method of the nitrogen oxide, the combination of absorptionclass and selection of the absorbent are very important. As the nitrogenoxide, there are nitrogen monoxide (NO), nitrogen dioxide (NO₂),nitrogen tetroxide (N₂ O₄), dinitrogen trioxide (N₂ O₃), dinitrogenpentoxide (N₂ O₅), etc. However, from a stability point of view undernormal conditions, it is the primary object to remove the nitrogenmonoxide and the nitrogen dioxide which are highly stable. In a wetabsorption by an aqueous solution, reaction of the water upon thenitrogen oxide is represented by the following reaction formulas.

    2NO.sub.2 (or N.sub.2 O.sub.4) + H.sub.2 O⃡HNO.sub.3 + HNO.sub.2                                                 (1)

    2hno.sub.2 ⃡h.sub.2 o + no + no.sub.2 (or 1/2 N.sub.2 O.sub.4) (2)

    2no + o.sub.2 →2no.sub.2                            (3)

    2no.sub.2 ⃡n.sub.2 o.sub.4                     (4)

as the nitrogen oxide is hardly absorbed by water, oxidation of thenitrogen oxide is a rate determining step in the absorption and removalmethod by most aqueous solutions. Even in the actual absorption, thereis a problem of how to improve the oxidation reaction speed. As themethods therefor, there can be enumerated an oxidation absorption methodby catalytic oxidation, a vapor phase oxidation method using vapor phaseoxidizing agents such as ozone, etc., a chemical reaction absorptionmethod using a compound reactive upon the nitrogen monoxide, and othermethods. If estimation in effected from the reaction formula, concerninga third method, the nitrogen monoxide is considered to allow thefollowing reaction to be applicable.

(a) reaction by sulfuric acid (H₂ SO₄)

    no + h.sub.2 so.sub.4 = h.sub.2 so.sub.4.no

(b) reaction by chlorine (Cl₂) and ammonia (NH₃)

    2no + cl.sub.2 = 2NOCl

    NOCl + 2NH.sub.3 = NH.sub.4 Cl + N.sub.2 + H.sub.2 O

(c) reaction by sodium chlorite (NaClO₂)

    4no + 3naClO.sub.2 + 4NaOH = 4NaNO.sub.3 + 3NaCl + 2H.sub.2 O

(d) reaction by permanganate (KMnO₄)

    2kmnO.sub.4 + 2NO + H.sub.2 O = K.sub.2 O + 2MnO.sub.2 + 2HNO.sub.3

(e) reaction by ferrous sulfate (FeSO₄)

    no + feSO.sub.4 = Fe (NO)SO.sub.4

generally, such reaction formulas as described hereinabove areconsidered, but they only show reaction possibilities. Practically, thereaction speed thereof is important. Accordingly, the inventors havemade laboratory tests to estimate the practical possibilities concerningapproximately thirty kinds of materials including the above-mentionedmaterials. The materials

NaOH, Ca(OH)₂, Mg(OH)₂, KOH, Na₂ CO₃, NaHCO₃, Na₂ SO₄, NaHSO₃, Na₂ S₂O₃, K₂ SO₄, FeSO₄, NaClO, NaClO₂, NaClO₃, FeCl₂, Ca(OCl)₂, NaIO₄, NaIO₃,NH₄ Cl, (NH₄)₂ SO₄, NH₄ SCN, NaSCN, H₂ O₂, KMnO₄, KMnO₄ + NaOH, K₂ Cr₂O₇, K₂ Cr₂ O₇ + H₂ SO₄. Solid materials have been dissolved in water,etc. to a concentration of 2 to 5%.

In this case, the apparatus shown in FIG. 1 has been used. Numeral 1 isa N₂ bomb, numeral 2 being a NO bomb of 1,000 ppm in NO concentration.Numerals 3 and 4 show a valve of each bomb respectively. Numeral 5 is amixer wherein N₂ and NO are mixed. Numeral 6 is an absorption bottlewherein absorption liquid 7 is accommodated. Numeral 8 is a water tankof constant temperature. Numeral 9 is a pipe which is dipped in theliquid 7 to effect bubbling operation, in the absorption liquid, ofmixed gas from the mixer 5. Numeral 10 is a cooler for cooling a gasguided out from the bottle 6. Numeral 11 is an analyzer for measuringthe concentration of NOx. The NOx concentration of the gas which entersthe absorption bottle 6 and the NOx concentration of the gas which hascome out of the absorption bottle 6 are measured by the NOx analyzer 11to investigate how much NOx has been absorbed and removed by theabsorption liquid 7. In the apparatus illustrated above, the NO gas isdiluted in the N₂ gas and is introduced into the absorption liquid 7 inthe absorption bottle 6 for bubbling operation thereof. The experimentalconditions are as follows.

Absorption liquid capacity: 400 (ml)

Absorption liquid temperature: 30 (° C.)

Gas flow: 1.0 (l/min)

No concentration at entrance: 90 (ppm)

In this experiment, a Kelminex method (Yanagimoto Manufactory of Japan,Yanaco ECL-7S) has been used to analyze the NOx of the nitrogen oxide.The results are shown in FIGS. 3 and 4 respectively. FIG. 3 shows theresults at 30° C., and FIG. 4 the results at 50° C. Referring to FIGS. 3and 4, reference character A shows a feature of a mixed liquid betweenK₂ Cr₂ O₇ and H₂ SO₄, reference character B showing that of a 5% aqueoussolution of NaClO₂, reference character C showing that of a 2% aqueoussolution of KMnO₄, reference character D showing that of an aqueoussolution of KMnO₄ (2%) and NaOH(4%), reference character E showing thatof a mixed liquid of K₂ Cr₂ O₇, H₂ SO₄ and H₂ O, reference character Fshowing that of a 5% aqueous solution of NaClO. Materials except thoseshown are completely free of removal capability, or have been reduced insuch capability in an extremely short time. From FIGS. 3 and 4, it isapparent that the 5% solution of NaClO₂ (B), and K₂ Cr₂ O₇ + H₂ SO₄ (A)are particularly superior, having a 100% removal rate for a long period.Results of service life testing concerning A and B are shown in FIG. 5.The experiments have been effected under the following conditions:

No concentration at entrance: 980 (ppm)

Absorption liquid capacity: 50 (ml)

Gas flow: 1 (l/min)

Absorption liquid temperature: 30 (° C.)

According to the comparison between FIGS. 3 and 4, the results show thathigher temperatures of the absorption liquid produce inferior absorptionefficiency. Perhaps, this is because, in the absorption reaction(formulas 1 to 4 described above) of the nitrogen oxide upon the water,the cracking reaction of nitrous acid (formula 2) is accelerated due totemperatures.

According to the results of FIGS. 3 and 4, from a reaction speed pointof view, the absorbents, in aqueous solution, are powerful oxidationagents, and the absorbent, which is considered to participate in thedirect reaction with the NO or the oxidation reaction therewith, showsits superior nitrogen oxide removal capability. If screening is effectedin these powerful oxidation agents from poison handling and safetypoints of view, both NaClO₂ and KMnO₄ are necessarily chosen. From theresults of FIGS. 3 and 4, K₂ Cr₂ O₇ + H₂ SO₄ solution is superior in NOremoval efficiency, but causes problems in safety and other points.Thus, it is not practical. Both NaClO₂ and KMnO₄ are considered to besuperior in NO removal efficiency, since the chemical reactions shown inc and d above proceed extremely quickly, together with the oxidationreaction of the NO. As moisture normally exists even in the combustionexhaust gases, and the normal indoor environment, it is possible toapply the concept of the conventionally known wet absorption method tothis new dry absorption method.

From this expression, a dry absorption composition with sodium chlorite(NaClO₂) or potassium permanganate (KMnO₄) used as the main componenthas been examined in construction. At first, mixture with the othermaterials wherein NaClO₂ is used as a main component of the mixture hasbeen examined in two kinds of methods. One of them is a method ofimpregnating the NaClO₂ with the other material as a carrier, and theother method is a method of mixing and scouring the other material withNaClO₂. The former impregnation method is a method which has beenembodied partly in the aforesaid Japanese Patent Publication No.10,048/1968, etc. However, the latter mixing and scouring method isunprecedented. The present inventors have examined through comparisons atwo component class of the NaClO₂ and the other material in theimpregnation method, and the mixing and scouring method respectively.Materials which have been examined through comparisons by theimpregnation method and the mixing and scouring method are in thefollowing groups.

Activated charcoal, various molecular sieves, silica gel, activatedalumina, dryerite, soda lime, plaster of Paris, bentonite, slaked lime,quick lime, calcium chloride.

The first six kinds of materials among the abovementioned materials areknown normally as the absorbents. However, according to the resultswherein they have been examined individually, they have proved to befree from NO removal capability at all.

In this case, the apparatus shown in FIG. 2 has been used. Referring tothe drawing, numeral 21 is a N₂ bottle. Numeral 22 is a NO bomb of 1,000ppm in NO concentration. Numerals 23 and 24 show a valve of each bottle.Numeral 25 is a mixture wherein N₂ gas and NO gas are mixed. Numeral 26is a quartz glass reactor which guides from an entrance 27 the mixed gasfrom the mixer 25 thereby to react upon samples 29 filled therein.Numeral 28 is an exit of the gas from the reactor 26. Numeral 30 is aheater for heating the reactor 26. Numeral 31 is a temperaturecontroller which controls current flowing to the heater 30 to keep thetemperature of the reactor 26 constant. Numeral 32 is a temperaturerecorder. Numeral 33 is an analyzer for measuring the concentration ofthe nitrogen oxide NOx. Numeral 34 is its recorder. In this apparatus,the NO gas of 1,000 (ppm) is diluted in N₂, and is mixed sufficiently bythe mixer 25, and then is introduced into the reactor 26. The NOxconcentration of the gas which has entered the reactor 26, and the NOxconcentration of the gas which exits the reactor 26 are measured by theanalyzer 33 to investigate the NOx removal efficiency of the samples 29.This has been carried out under the following conditions.

Temperature of adsorption layer 29: 30, 50, 100 (° C.)

Adsorption layer volume: 20 (ml)

Gas flow: 5.5 (l/min)

Space Velocity (SV value): 16,500 (h⁻¹)

Classes wherein sufficient capabilities for removing

the nitrogen monoxide have been obtained are as follows:

(1) in a case where 5% NaClO₂ aqueous solution has been impregnated intoplaster of Paris (CaSO₄.1/2 H₂ O),

(2) in a case where 50% NaClO₂ solution has been impregnated intoactivated alumina,

(3) in case where plaster of Paris (CaSO₄) and NaClO₂ have been mixedand scoured in the rate of three to two, and

(4) in a case where slaked lime [Ca(OH)₂ ] and NaClO₂ have been mixedand scoured at the rate of three to two.

There are intrinsic optimum compositions relative to the impregnationconcentration of NaClO₂ and mixing scouring ratio thereof in the fourkinds of construction. The values shown herein have proved to becompositions by which the highest capabilities thereof are obtained.FIG. 6 shows the capability comparisons. In the experiment of FIG. 6,the exhaust gas (NO: 170 ppm) resulting from propane gas burnt by meansof a Bunsen burner has passed under space velocity of SV = 12,000 hr⁻¹and temperature of 200° C. Referring to FIG. 6, A shows features ofpaster of Paris and NaClO₂ mixed and scoured in the rate of three to two(weight ratio), B features of the plaster into which NaClO₂ 5% aqueoussolution has been impregnated, C the features of activated alumina intowhich NaClO₂ 50% aqueous solution has been impregnated. As apparent fromFIG. 6, the mixing and scouring class A between the plaster of Paris andthe sodium chlorite has very superior NO removal capabilities ascompared to the conventional impregnation type C. FIG. 7 shows effectsapplied upon the time lapse variations of the NO removal rate in a casewhere the mixing ratio has been changed in three kinds of mixing andscouring operations, namely class A of plaster of Paris (namely, calciumsulfate) and sodium chlorite, class B of slaked lime (namely, calciumhydroxide) and sodium chlorite and class C of plaster of Paris, slakedlime and sodium chlorite. The mixing ratios are all shown in weightratio. As apparent from FIG. 7, in the plaster of Paris/sodium chloriteclass A, it is desirable that the mixing ratio of the plaster ofParis/sodium chlorite is in the range of 100/1 to 1/10. Likewise, evenin the slaked lime/sodium chlorite class B, it is desirable that themixing ratio of the slaked lime/sodium chlorite is in the range of 100/1to 1/10. Also, the three-component mixing and scouring class C of theplaster of Paris/slaked lime/sodium chlorite has much better NO removalcapability than the two-component class. FIGS. 8 and 9 show the effectsof the composition ratio upon the NO removal rate concerning thethree-component mixing and scouring system of the plaster ofParis/slaked lime/sodium chlorite. In FIG. 8, (plaster of Paris + slakedlime): NaClO₂ is fixed to 3:2, and the mixing percentage of the plasterof Paris and the slaked lime is changed. In FIG. 9, plaster ofParis:slaked lime is fixed to 1:1, and the sodium chlorite is changedwith plaster of Paris + slaked lime as 3. Entrance concentration of thegas introduced is NO = 85 ppm, space velocity being SV = 16,500 hr⁻¹,and temperature being 30° C. It has been found that the highestcapability is obtained in the composition of 3/3/4 by weight ratio asthe composition ratio of plaster of Paris/slaked lime/sodium chlorite.As apparent from these, the mixing ratio in a case where the calciumhydroxide and the calcium sulfate are mixed is optional in thethree-component class C. The mixing ratio between the sodium chloriteand a mixture of the calcium hydroxide and the calcium sulfate isdesirable to be in the range of 100/1 to 1/10 by weight.

An aqueous solution of sodium chlorite reacts upon nitrogen monoxide asshown in the following formula.

    4NO + 3NaClO.sub.2 + 4NaOH = 4NaNO.sub.3 + 3NaCl + 2H.sub.2 O

At this time, the calcium sulfate and the calcium hydroxide serve as acarrier for carrying the sodium chlorite and act to improve the contactefficiency of the sodium chlorite and NO due to larger specific surfacearea. Also, especially the calcium hydroxide is alkaline, and isconsidered to absorb and remove the NO₂ of an intermediate product whichhas not been reacted sufficiently by reaction of the sodium chlorite andNO.

FIG. 10 shows the results of the capability for removing NO throughapplication of the composition of the present invention to combustionexhaust gas. The exhaust gas produced by burning of propane gas by meansof a Bunsen burner is brought into contact with the composition of thepresent invention under conditions of SV = 12,000 (hr⁻¹) and packedlayer temperature of 200 (° C.). The NOx concentration in the exhaustgas is 180 (ppm), and the NO accounts for 95% or more thereof. Referringto FIG. 10, A shows the calcium sulfate and the sodium chlorite mixed inthe rate of three to two by weight ratio, B showing the calciumhydroxide and sodium chlorite mixed in the rate of three to two, Cshowing the calcium hydroxide, the calcium sulfate, and the sodiumchlorite mixed in the rate of three to four. And, D is the feature ofthe conventional example wherein a 50% aqueous solution of the sodiumchlorite has been impregnated into activated alumina. Furthermore, A, Band C are used wherein each component is blended, mixed and scoured withaddition of water, moulded into proper shape, dried, crushed and siftedout into 6 to 8 mesh. As apparent from FIG. 10, the A, B and C of thepresent invention have remarkably superior NO removing capabilities ascompared with the conventional composition D.

The relationship between the mixing ratio and the NO removal ratio hasbeen examined for KMnO₄ /CaSO₄, KMnO₄ /Ca(OH)₂ and KMnO₄ /Ca(OH)₂composition relative to KMnO₄. The results thereof are shown in FIG. 11.Referring to FIG. 11, A shows a class of KMnO₄ /Ca(OH)₂ /CaSO₄, Bshowing a class of KMnO₄ /Ca(OH)₂, C showing a class of KMnO₄ /CaSO₄.The NO concentration of the gas used is 85 ppm, the space speed SV being17,000 hr⁻¹, the temperature being 30° C. In FIG. 11, it is desirablethat the mixing ratio of potassium permanganate and calcium sulfateshown by C is a ratio of 1/500 to 1/1 by weight if 70% initial removalratio is desired, the mixing ratio of the potassium permanganate andcalcium hydroxide shown by B being a rate of 1/400 to 1/1, the mixingratio of potassium permanganate with the mixture of calcium hydroxideand calcium sulfate shown by A being a ratio of 1/500 to 1/1 (the mixingratio of calcium hydroxide and calcium sulfate is optional).

FIG. 12 shows the NO removal capability in a case where each compositionof A, B and C shown in FIG. 11 has been applied to combustion exhaustgas. Also, D in FIG. 12 represents the conventional composition whereina 5% KMnO₄ solution has been impregnated into activated alumina carrier.In this experiment, the exhaust gas produced by burning propane gas byuse of a Bunsen burner has been passed in contact with the packed layerof the composition under the conditions of space velocity SV = 12,000(hr⁻¹). The NO concentration of the exhaust gas has been 180 (ppm), thepacked layer temperature 200 (° C.). Also, FIG. 13 shows results of theNO removal rate obtained with the compositions A, B, C and D bycontacting the nitrogen monoxide of 85 ppm with the compositions, thenitrogen monoxide being nitrogen-balanced and containing saturated steamat a temperature of 30° C. under the conditions of space velocity SV =17,000 (hr⁻¹). It is apparent from FIGS. 12 and 13 that the compositionsA, B and C of the present invention are superior in NO removalcapability as compared with the conventional composition D. Also, amongthe compositions A, B and C, the calcium hydroxide mixing and scouringclasses A and B are provided with better performance than the calciumsulfate mixing and scouring class C. Comparison between FIGS. 12 and 13shows that each composition A to C is better in NO removal rate if it isused for application to exhaust gas. Namely, this shows that thecomposition of the present invention is more practical.

As described hereinabove, the aqueous solution of the potassiumpermanganate is considered to react upon NO as follows.

    2KMnO.sub.4 + 2NO + H.sub.2 O = K.sub.2 O + 2MnO.sub.2 + 2HNO.sub.3

the calcium sulfate and the calcium hydroxide are considered to serve tocarry the KMnO₄ to act to absorb, as alkali, the NO₂ existing in thereaction, although this is not clear, or to act to adsorb it.

In order to make the compositions of the present invention suitable forpractical use, it is necessary to granulate them in large quantities instable quality and shape. There are generally enumerated crushinggranulating, extruding granulating, transiting granulating, bricketing,etc. In the compositions of the present invention, the granulatingmethod is restricted to a special one in order to exhibit sufficientefficiency thereof. In the compositions of the present invention, theextruding method, among the granulating methods, is most desirable. Inextruding granulating, the following steps are preferable as a processto retain the compositions of the present invention at a high level ofperformance.

(1) mixing of materials→(2) grinding→

(3) kneading→(4) extruding→(5) drying

The reason why the extruding granulating is preferable as thegranulating method is based on the accumulation of experimental dataobtained through actual comparison, in NO removal capabilities, ofproducts made by each kind of granulating method. Table 1 shows oneportion of the data. Each method has used the calcium hydroxide, calciumsulfate, and sodium chlorite mixed in a ratio of three to three to fourby weight. The NO concentration of the gas used is 1,000 ppm, the spacevelocity SV being 16,000 hr⁻¹. The removal ratio of the NOx in the tableis an average value of fifteen minutes from its initial stage. In theextruding granulating wherein a mixed solvent of water (H₂ O)/ethanol(C₂H₅ OH) is used as a binder, as apparent from Table 1 superior NO removalcapability is obtained. In the crushing granulating, unfavorable rate ofproduce yield is particularly a problem. Accordingly, this isunacceptable as an industrial method. Also, in the bricketing method,the feature of the materials themselves becomes a problem in terms offluidity and lubricating, thus resulting in difficult mass production.

                                      Table 1                                     __________________________________________________________________________                              NOx removing rate (%)                                                         (initial stage)                                     granulating                                                                          granulating conditions                                                                           NO remov-                                                                            NOx remov-                                                                           specific surface area                 method binder    others   ing rate                                                                             ing rate                                                                             (m.sup.2 /g)                          __________________________________________________________________________           H.sub.2 O kneader peri-                                                                          10      7     1.33                                                   od : 5 min.                                                  extruding type gra- nulating                                                          ##STR1## grinding per- formed kneader peri- od : 15                                             100    75     7.58                                         H.sub.2 O kneader peri-                                                                          10      7     2.22                                                   od : 5 min                                                   transiting type gra- nulating                                                         ##STR2## kneader peri- od: 5 min.                                                               50     32     2.43                                         H.sub.2 O kneader peri-                                                                 od : 15 min.                                                                           20     15     6.04                                  crushing type gra- nulating                                                           ##STR3## kneader peri- od : 15 min.                                                             100    63     4.66                                  bricketing                                                                           filler material such as                                                       magnesium stearate etc.                                                                          100    65     7.20                                         is used                                                                __________________________________________________________________________

Particularly, when only water has been used as a binder to be added inthe kneading process, the desired capability is difficult to obtain.Accordingly, it is necessary to adopt the mixed solvent ofwater/alcohol. FIG. 14 shows the variation of the NOx removal capabilitywhen the mixing ratio (capacity ratio) of the water and the ethylalcohol has been changed. The gas used here is 100 ppm in NOconcentration with NO and air being balanced, and the space speed SV = 5× 10⁴. Each measured value is a fifteen minutes' average value from theinitial stage. It is apparent from FIG. 14 that the ethanol improves thecapability thereof. However, actually, it is most desirable that themixing ratio of the water/ethyl alcohol is near 1/1 by capacity ratio.

Although the requirement of the grinding process is related to thematerial grain size used, the capability thereof is improved if thegrinding process is provided. The effects of the grinding process uponthe NOx removal capability are shown in Table 2. The calcium hydroxide,calcium sulfate and sodium chlorite composition mixed at a ratio ofthree to three to four by weight ratio has been used in all cases. Themeasuring conditions are the same as in Table 1.

                  Table 2                                                         ______________________________________                                        extruding                                                                     conditions                       specific                                     kneading                NOx removal rate                                                                           surface                                  period       grinding by                                                                              (initial stage)                                                                            area                                     no. binder  (min.)   hammer mill                                                                            NO     NOx   (m.sup.2 /g)                       ______________________________________                                            water/                                                                        ethanol                                                                   (1) = 1/1    2       no       100    79    5.97                               (2) "       15       no       100    82    3.77                               (3) "        2       yes      100    84    6.11                               (4) "       15       yes      100    89    8.07                               ______________________________________                                    

From the results of Table 2, it is apparent that the grinding process,in the extruding granulating process for the compositions of the presentinvention, provides favorable results to capability improvement. Thereasons why the grinding process improves the NOx removal capability,and the extruding granulating is best as the granulating method, areconsidered to be as follows. One of the reasons is that the differencein the specific surface area obtained is involved as shown in Tables 1and 2. Namely, it is advantageous during reaction if the specificsurface area is larger. Furthermore, although it is an estimate, thepatterns of pore distribution of the granulated products are different(in two or three examples actually measured such distributions have beendifferent), depending upon the granulating method and the granulatingconditions. When gas and solid have been brought into contact with andreacted upon each other on a fixed bed, differences are caused in thediffusion of the NOx content into the granulated products and thus thereactivities thereof are different. These differences are considered tohave influences upon the facts described hereinabove. Furthermore,reactivities, these influences upon the activity itself, and otherfactors are considered to have influence upon the facts describedhereinabove.

FIG. 15 shows the comparison of features of the NOx removal capabilitieswith those of the conventional example, the extruding granulatedproducts of the compositions of the present invention being constitutedas air purifying filters. In all cases, the filter is 15 mm thick, andthe area is 0.09 m². The gas used is about 1 ppm in NO concentration,and is about 1 ppm in SO₂ concentration. These removal rates have beenmeasured by changing the blowing amount. The product A of the presentinvention uses the composition mixed at a ratio of three to three tofour (weight ratio) of Ca(OH)₂, CaSO₄ and NaClO₂ and is granulated intopellet shape of 2.5 mm in diameter by the extruding process describedhereinbefore. As the granulating conditions, the grinding is performed,the binder of the water/ethanol = 1/1 being used, the kneading periodbeing 15 minutes. The conventional product B used activated charcoal. Asshown in FIG. 15, the composition of Ca(OH)₂ /CaSO₄ /NaClO₂ = 3/3/4 canremove sulfurous acid gas SO₂, in addition to NO, and has superiorremoval capabilities. From an air cleaning point of view, removal of thesulfurous acid gas has been an important demand. Accordingly, thepresent product can satisfy the requirements sufficiently.

Also, the composition of the present invention may be constituted as afilter by being carried on a porous structure. As the porous structureused therefor, there can be enumerated assemblies of fine wires such asglass wool, stainless, etc. In structure, it is possible to realize theconstruction such as net shape, honey comb shape, ball shape, etc. Inany event, it is desirable to have very porous construction. There aremany ways of carrying the composition onto the porous constructionmember. For example, in case of the Ca(OH)₂ /CaSO₄ /NaClO₂ composition,there are two methods, one method wherein the calcium sulfate and thecalcium hydroxide are subjected to hydration and kneading in advance andrendered fluid, being applied and dried on the porous constructionmember, thereafter being brought into contact with an aqueous solutionor a suspension of the sodium chlorite to impregnate the sodiumchlorite. The other method is one wherein a third member is applied in afluid condition after hydration and kneading. The latter is morepreferable than the former, because the adhesion amount of the sodiumchlorite onto the porous construction member is more in the latter. Suchcomposition of the present invention carried on the porous constructionmember has advantages in that there is a smaller loss of pressure, whichis extremely advantageous, in the case of applying air filtration to alower extent. Secondly, sufficient mechanical intensity is ensured.Thirdly, NO removal capabilities can be improved by possible increase ofthe specific surface area up to approximately ten times as much bycombination of the present composition with the construction member,although the composition of the present invention is on the order of 10(m² /g).

An embodiment of this feature of the invention will now be described. Asthe porous construction member, a urethane blister member is used. Afilter wherein a mixture in fluid condition, in which Ca(OH)₂ /CaSO₄/NaClO₂ = 3/3/4 (weight ratio) is subjected to hydration and kneading,is adhered onto the urethane blister member. FIG. 16 shows the featuresof the filter. In the graph, reference character A is the feature of thefilter, reference character B being the feature of the packed layerwhich is composed of the granulated products wherein the composition hasbeen formed into pellet shape of 2.5 mm in diameter. The filter is 15 mmin thickness, and the area is 0.09 m². The gas of approximately 1 ppm inNO concentration is used. The filter A is slightly lower in NO removalrate as compared with the packed layer B, but is characterized as havingsmaller pressure loss. For example, in adding harmful gas-removingcapabilities to an air conditioner for cooling and heating, the coolingand heating capabilities are not damaged comparatively by use of thefilter. As a composition of the sodium chlorite has been used forcleaning indoor air, and has been brought into contact with a largeamount of air, some chlorine smell may be noticeable. This becomes a bigdisadvantage when air purification is desired. Accordingly, acountermeasure against the chlorine is compulsory in this instance. Asthe countermeasure, a chlorine removing agent may be used. The exampleswhere activated charcoal has been used are shown in Table 3.

                                      Table 3                                     __________________________________________________________________________                NOx removal capability                                                                      chlorine concentration                                          NO removal                                                                           NOx removal                                                                          analyzed by existence                               filter construction                                                                       rate   rate   O-tolidime method                                                                         of smell                                __________________________________________________________________________    CaSO.sub.4 /NaClO.sub.2 = 2/3                                                             100    68     5 p.p.m.    yes                                     Ca(OH).sub.2 /NaClO.sub.2                                                     = 2/3       100    70     1 p.p.m.    yes                                     CaSO.sub.4 /Ca(OH).sub.2 /                                                    NaClO.sub.2 = 3/3/4                                                                       100    85     0.5 p.p.m.  yes                                     combination of                                                                activated charcoal                                                            with CaSO.sub.4 /                                                             Ca(OH).sub.2 /NaClO.sub.2                                                                 100    95     not detected                                                                              no                                      __________________________________________________________________________

Combination thereof with a removing agent which is composed of alkali,etc. such as sodium carbonate, without the activated charcoal ispossible to use. As apparent from FIG. 3, when it has been combined withthe activated charcoal, NOx removal capability is further improved dueto the multiplication effect of the present composition with theactivated charcoal, since the activated charcoal is also comparativelysuperior in NO removal capability. The combination thereof with theactivated charcoal is preferable not only for prevention of the chlorinesmell, but also for improvement of the NOx removal capability.

As described hereinabove, the present invention results in a greaterrate of and capability for removing nitrogen oxide from gas,particularly, nitrogen monoxide. Therefore, cleaning of the waste gasproduced by burning can be improved remarkably. Furthermore, thecomposition of the present invention can be in the form of a packedlayer or can be carried on a porous construction member. Accordingly, itcan easily be of a dry type, and is convenient in use. As constructionof the gas cleaning apparatus can be simplified, the present inventionis very useful. Furthermore, since even nitrogen oxide of lowconcentration in the atmosphere or environment can be better removed,the present invention is very helpful for cleaning the air.

What is claimed is:
 1. A granulated gas-purifying composition comprisingcalcium sulfate and potassium permanganate.
 2. A granulated,gas-purifying composition comprising a mixture of calcium sulfate andsodium chlorite, the weight ratio of the calcium sulfate:the sodiumchlorite being in the range of 100:1 to 1:10.
 3. A granulated,gas-purifying composition comprising a mixture of calcium sulfate,calcium hydroxide and sodium chlorite, the weight ratio of the total ofthe calcium sulfate and the calcium hydroxide:the sodium chlorite beingin the range of 1:100 to 10:1.
 4. A granulated, gas-purifyingcomposition comprising a mixture of calcium sulfate and potassiumpermanganate, the weight ratio of the calcium sulfate:the potassiumpermanganate in the range of 500:1 to 1:1.
 5. A granulated,gas-purifying composition comprising a mixture of calcium sulfate,calcium hydroxide and potassium permanganate, the weight ratio of thetotal of the calcium sulfate and the calcium hydroxide:the potassiumpermanganate being in the range of 500:1 to 1:1.
 6. A method ofproducing a granular composition for gas purification, which comprisesmixing (1) at least one member selected from the group consisting ofcalcium sulfate and calcium hydroxide with (2) an oxidizing agent havingsufficient oxidizing capability to react upon NO, grinding the resultantmixture, adding water and alcohol to the ground mixture to knead themixture, and extruding the kneaded mixture through a mold to granulatethe mixture.